The present invention relates generally to a technology of making a judgement about a fault in an exhaust gas purifying catalyst of an internal combustion engine of an automobile and the like and, more particularly, to a technology of making a judgement about a fault in an exhaust gas purifying apparatus which includes an exhaust gas purifying catalyst covered with an adiabatic material and a heat storage material.
The internal combustion engine of the automobile and the like has an exhaust gas purifying catalyst carrying as a catalyst a precious metal such as a platinum, palladium and the like in order to purify harmful components contained in the exhaust gas discharged, such as, e.g., carbon monoxide (CO), nitrogen oxide (Nox) and hydro carbon (HC) before these components are discharged into the atmospheric air.
The exhaust gas purifying catalyst makes, for example, HC and CO contained in the exhaust gas react to oxygen O.sub.2 in the exhaust gas, which are thus oxidized into H.sub.2 O and CO.sub.2. At the same time, the exhaust gas purifying catalyst makes Nox in the exhaust gas react to HC and CO in the exhaust gas, which are thus reduced to H.sub.2 O, CO.sub.2 and N.sub.2.
The exhaust gas purifying catalyst is activated at a predetermined temperature or higher and brought into an inactive state when lower than the predetermined temperature. Therefore, when a temperature of the exhaust gas purifying catalyst is lower than the predetermined temperature as in the case of the internal combustion engine being cold-started up, there might be such a problem that the harmful components in the exhaust gas can not be completely eliminated.
What is known as a device for obviating such a problem is the catalyst device disclosed in SAE Technical Paper #961134, #950409, #941998. This catalyst device includes a flame tube incorporating the exhaust gas purifying catalyst, a heat storage material covering the flame tube, an intermediate tube covering the heat storage material, an outer casing covering the flame tube and the inter mediate tube, a vacuum spatial portion formed between the flame tube, the intermediate tube and the outer casing, and a hydrogen absorbing alloy disposed in the vacuum spatial portion. During an operation of the internal combustion engine, the vacuum spatial portion is set in a non-vacuum state by emitting the hydrogen out of the hydrogen absorbing alloy, thereby transmitting heat of the exhaust gas purifying catalyst via the hydrogen to the outer casing and restraining an excessive rise in temperature of the exhaust gas purifying catalyst. After a halt of the operation of the internal combustion engine, the hydrogen is absorbed by the hydrogen absorbing alloy, thereby setting the vacuum spatial portion in a vacuum state. Heat radiations from the exhaust gas purifying catalyst and the heat storage material are thus cut off by the vacuum spatial portion, and the activated state of the exhaust gas purifying catalyst is maintained till the engine is started next time.
In the thus constructed catalyst device, it is also of importance to precisely diagnose a fault in the catalyst device. For example, if a hole or a crack is caused in the outer casing or the intermediate tube or the flame tube, and if the hydrogen absorbing alloy is deteriorated, there might be a decrease in degree of vacuum of the vacuum spatial portion, a deterioration and an outflow of the heat storage material or deteriorations of a hydrogen absorbing performance and a hydrogen emitting performance, resulting in declines of an adiabatic performance and a heat radiating performance of the catalyst device.
If the adiabatic performance of the catalyst device declines, it is impossible to cut off the heat radiations from the exhaust gas purifying catalyst and of the heat storage material, and it is difficult to keep the exhaust gas purifying catalyst at the activation temperature or higher after the halt of the internal combustion engine, with the result that an exhaust emission worsens when started next time.
Further, if the heat radiation performance of the catalyst device declines, the heat of the exhaust gas purifying catalyst and of the heat storage material can not be transmitted to the outer casing and the like, which leads to an excessive rise in temperature of the exhaust gas purifying catalyst with a decrease in durability of the exhaust gas purifying catalyst.